It is proposed to continue and extend a project on long range excitation transport and penetration in semiordered molecular aggregates. Most experimental studies will be performed on simple, synthetic prototype systems, each with several pi-system components (naphthalenes, porphyrins, etc.), where the concentration is the orderparameter. Specifically, we propose to investigate both theoretically and experimentally the dependence of the exciton percolation (catastrophic transfer) on the interaction topology, the excitation lifetime, jump-time and coherence time, as well as the aggregate size, shape and its boundary conditions. This will include excitation percolation on thin films and interfaces and energy percolation through membranes. The roles of molecular exciton tunneling, fission and fusion as well as exciton-phonon interactions will also be studied for these semiordered aggregates, utilizing state-of-the-art laser excitation, i.e., monochromatic, tunable, coherent and pulsed (nanosecond) light, in conjunction with externally produced electric field gradients. These studies should result in detailed information on the switching ("on and off") of the excitation transport in the studied prototype systems, as well as for in-vivo aggregates, under both normal and abnormal conditions. Short range applications involve the primary physical process of photosynthesis, and possibly, muscle contraction, mitochondrial respiration and neuron excitation. The long range goal is a model for a "computer" based on molecular excitons and its application to damaging processes affecting the brain and nervous system.